Design of The Tank Circuit We have shown that the conversion efficiency from D.C. to H.F. is determined by the type of anode current wave used, and on the correct impedance of the loaded tank circuit to utilize the peak emission of the valve under the conditions appertaining, i.e. whether Class B or Class C. It is, however, equally important to design the tank so that its losses are the least possible. If the output is coupled through a mutual inductance M, and the load circuit is in resonance, we have: w M Icc = R0 I0 The power in the load is I02 R1 or in the terms of primary current, w2 M2 Icc2 / R0 and the power input to the tank circuit is Icc2 ( Rcc + w2 M2 / R0) The ratio power-transferred-to-the-load / power-input-to-tank-circuit may be termed the transfer efficiency of the tank circuit and is evidently given by (w2 M2 / R0) / (Rcc + w2 M2 / R0) If Q1 and Q2 are the "Q values" of the tank circuit when unloaded and loaded respectively, then Q1 = w L / Rcc and Q2 = w L / (Rcc + w2 M2 / R0) Hence w2 M2 / R0 = w L ((1/Q2) - (1/Q1)) and transfer efficiency may be written as Q2 ((1/Q2) - (1/Q1)) or (Q1 - Q2 / Q1) 100 as a percentage. From the above it is seen that the greater the difference between Q1 and Q2 the greater the transfer efficiency. Assuming the unloaded tank circuit Q1 is fixed irrespective of its L.C. ratio ( a justifiable assumption), although Q2 should be made as low as possible to obtain the highest transfer efficiency, in practice the minimum value of Q2 depends upon a number of factors. The lower we make Q2, the greater the harmonic content as previously explained, and the less the coincidence between maximum output and unity power factor. On the other hand, an increase of Q2 reduces the transfer efficiency and if increased too much may introduce "sideband cutting." Actual values of Q2 range from 15 to 20 on small power sets down to 3 and 4 on larger sets, and transfer efficiencies become possible up to values as high as 95%. Although it is not obvious from the above formula, we obtain the greatest transfer efficiency by increasing the L/C ratio to the maximum possible, but here again the type of circuit will probably determine the actual design and L/C used. For instance, consider a triode transmitter designed to give an output of 1 kW on a minimum wavelength of 15 meters from a D.C. supply voltage of 5,000. The valve and stray capacitances of the output circuit (including that of a small tuning capacitance set to minimum) to deliver this power could easily be as much as 20uuF. Thus with no additional deliberate tuning capacitance we should need an inductance of only 3.16uH to tune to 15 meters. If we assume the minimum anode voltage is 1,000, then the R.M.S. value of the A.C. anode voltage is 2,830 volts and we have Icc = E w C = 7.13 amps. Now Q2 = V A / watts = 7.13 2,830 / 1,000 = 20.2 This is the minimum Q2 value that can be obtained with a circuit layout and valve having this particular stray capacitance, operating on the voltage mentioned.